Advances in making large-capacity optical disk devices have led to products up to now including compact disks (CD) utilizing infrared LD (wavelength 780 nm), DVD utilizing red LD (wavelength 650 nm), and Blu-ray discs (BD) utilizing blue LD (405 nm).
The write strategy technology in the BD-RE (Rewritable) for example utilizes multiple LD pulses possessing up to three types of power levels as shown in FIG. 2. These three types of power levels in order of high power are write power (Pw), erase power (Pe), and bottom power (Pb). Irradiating the write power (Pw) LD light onto the optical disk melts the recording film on the optical disk. After quickly cooling, the optical disk then reaches an amorphous state (non-crystallized state) where the light reflectance is low. This section of film is utilized as the recording mark. Moreover, irradiating the erase power (Pe) LD light onto the optical disk forms a crystallized state on the optical disk recording film. The section of the optical disk that was a non-crystallized state before irradiating the LD light reaches a crystallized state, and the section of the disk that was originally crystallized, remains in that state. The recording mark is erased in this way.
The write strategy includes rectangular mono-pulses utilized in CD-R, DVD-R, comb-like multi-pulses utilized in CD-RW, DVD-RW, DVD-RAM (See FIG. 2), and castle-shaped (non-multi) pulses utilized in high-speed recording of DVD optical disks (See FIG. 3).
As shown in FIG. 2 and FIG. 3, the values that determine the recording waveform edge timing, and values that determine recording power such as the write power Pw, gap power Pg, erase power Pe are called the recording parameters. Optimum values for these recording parameters are determined beforehand and prerecorded into each recording medium. In the BD-RE (rewritable Blu-ray disk) for example, the optimal recording parameter values are written into the DI information within the Permanent Information and Control data formed on the lead in zone on the inner circumference of the disk. Optimal values for recording parameter groups differ according to the composition and material used in the recording medium.
Namely, during recording of the recording marks, the recording parameters such as values for determining the recording waveform edge timing, values for determining recording powers such as Pw and Pg, and values for each shift table are loaded from the recording medium and an ideal recording mark is then formed by utilizing these loaded recording parameters to regulate the LD pulse.
A poorly formed recording mark might make correctly reproducing the recorded data impossible so the recording mark must be correctly formed. In order to form a satisfactory recording mark, the LD emits pulsed light to control the heat accumulating from light emitted onto the recording film of the optical disk. Electrical current is usually provided in a pulsed state to the semiconductor laser diode (hereafter called LD) that emits the LD light.
The LD driver (LDD) is a device for providing electrical current in a pulsed state to the LD. Providing electrical current in a pulsed state to the LD, by way of the LD driver allows the LD to generate a light emission pattern based on the pulse timing of the supplied electrical current, and in this way form a satisfactory recording mark.
The recording speed onto the optical disk tends to become faster every year. Next generation BD (Blu-ray disks) will also provide even better performance because of their multiple layers. Both of these factors involve even higher LD power.
The increased LD power is achieved by boosting the electrical current the LD driver supplies to the LD. The increase in electrical current that the LD driver supplies to the LD causes the LD driver (especially the LD drive unit) to consume more electrical current. A large change in the resistance value due in particular to the temperature is known to occur in the blue LD utilized in BD (blu-ray disks). FIG. 4 shows an equivalent LD circuit. This LD possesses a direct current resistive component R (hereafter called LD resistance) as shown in FIG. 4. During low temperatures the LD resistance tends to increase and also tends to decrease as the temperature rises as shown in FIG. 5. The relation between the LD drive current and the LD drive voltage (Hereafter called “LD-IV characteristic”) generated at both ends of the LD is as shown in FIG. 6. In other words, the LD-IV characteristic drops as the temperature rises. Maintaining the same LD drive current therefore requires lowering the LD drive voltage when the temperature becomes high. To lower the LD drive voltage, the technology described in JP-A 2007-334972 (=US2007/0291802) contains a pre-stored voltage value to supply to the laser driver when a specified temperature is reached, and attains low power consumption by regulating the voltage supplied to the power supply based on this stored voltage value.